This invention relates generally to chrominance signal (chroma) processing circuits and specifically to a chroma processing circuit and method for a digitized chroma signal. NTSC type color signals have a chroma bandwidth that is quite narrow in comparison with the luminance signal (luma) bandwidth. Chroma noise is quite noticeable to the viewer, especially in flat color areas. The limited frequency bandwidth of the chroma signal produces relatively slow chroma transitions which are also not desirable since they detract from sharp color demarcations. There have been numerous attempts in the prior art to enhance slow chroma transitions by adding additional signals, such as a second differential signal. The conflicting requirements of low noise and fast transitions have resulted in many design compromises.
In digital television systems, the chroma signals take the form of discrete pixels. Digital processing may be performed on these digital signals to perform various enhancement techniques. The present invention derives a first difference signal to indicate activity or change in the chroma signal. A second difference signal enhances or sharpens these changes or transitions when added to the original chroma signal. Groups of pixels are sampled and a median, a minimum and a maximum are determined. The median signal is supplied when the first difference signal is below an established threshold and the transient enhanced signal is supplied when the signal is above the threshold. The minimum and maximum signals are used to limit the undershoot and overshoot of the transient enhanced signal. Consequently, the processed color signal is optimized by either being noise reduced or transient enhanced. The invention will be seen to embrace a number of different aspects, namely, selectively noise reducing and transient enhancing a chroma signal; developing a transient enhanced chroma signal with controlled undershoot and overshoot characteristics; and the combination of a multiplexed chroma signal that is either noise reduced or transient enhanced with maximum and minimum limits for controlling overshoot and undershoot.